Technical Field
The present disclosure relates to semiconductor nanoparticles, a method of producing the semiconductor nanoparticles, and a light-emitting device using the semiconductor nanoparticles.
Description of Related Art
White light-emitting devices, which are used as backlights of a liquid crystal display devices and the like and which utilize photoluminescence emission from quantum dots (also called “semiconductor quantum dots”), have been proposed. Fine particles of semiconductor with a particle size of 10 nm or less, for example, are known to exhibit a quantum size effect. Such nanoparticles are called the quantum dots. The quantum size effect is a phenomenon where a valence band and a conduction band, each of which is regarded as continuous in bulk particles, become discrete when the particle size is on the nanoscale, whereby a bandgap energy is varied in accordance with their particle size.
Quantum dots absorb light and emit light corresponding to the bandgap energy thereof. Thus, quantum dots can be employed as a wavelength conversion material in light-emitting devices. Light-emitting devices using quantum dots are proposed, for example, in Japanese Unexamined Patent Application Publication No. 2012-212862 and Japanese Unexamined Patent Application Publication No. 2010-177656. More specifically, a portion of the light emitted from a light-emitting diode (LED) chip is absorbed by quantum dots, and photoluminescence of another color is emitted from the quantum dots. The photoluminescence emitted from the quantum dots and the light from the LED chip not absorbed by the quantum dots are mixed to produce white light. In these patent application documents, use of quantum dots made of a Group 12-Group 16 material, such as CdSe or CdTe, or a Group 14-Group 16 material, such as PbS or PbSe, is proposed. In International Patent Application Publication No. WO 2014/129067 A, a wavelength conversion film in which core-shell semiconductor quantum dots that does not contain Cd or Pb in view of toxicity of these elements is proposed. The formation of such a core-shell structure is also described in Chemical, Communications. 2010, vol. 46, pp 2082-2084.
One of the advantages of using quantum dots in light-emitting devices is that light with a wavelength corresponding to a bandgap of the quantum dots can have a peak with a relatively narrow full width at half maximum. Of the quantum dots proposed as the wavelength conversion material, quantum dots made of a binary semiconductor, typified by Group 12-14 semiconductors, such as CdSe, is confirmed to emit the light with the wavelength corresponding to the bandgap of the quantum dots, i.e., band-edge emission. Meanwhile, ternary quantum dots, in particular, Group 11-13-16 quantum dots have not been confirmed to exhibit the band-edge emission.
The light emission from the Group 11-13-16 quantum dots is caused by the defect levels at the surface or inside of the particles, or by the donor-acceptor-pair recombination, so that such light has a broad photoluminescence peak with a wide full width at half maximum and a long photoluminescence lifetime. Such light emission is not suitable for light-emitting devices, particularly, used in liquid crystal display device. This is because a light-emitting device used in a liquid crystal display device is required to emit light with a narrow full width at half maximum that has a peak wavelength corresponding to each of three primary colors (i.e., RGB) in order to ensure high color reproducibility. For this reason, practical application of the ternary quantum dots has not been prompted despite its less toxic composition.